1. Brief Description of the Invention
The invention described herein involves improved thickening compositions, often referred to as rheological additives, viscosity modifiers or thixotropes, used to impart Theological and viscosity modification properties to a wide range of aqueous or water-based systems. In many commercial applications, a thickening chemical such as a polyurethane polymer is added to an aqueous system, for example latex paint, to change or modify the Theological properties of the system.
Rheological additives of a solid powder type have proved difficult to mix or disperse into most aqueous systems. Dispersing these products into latex paints and coatings and other water-based systems is particularly burdensome. In view of this difficulty, such solid additives have been disfavored, and have not reached or approached their full market potential. To avoid dispersibility problems, some manufacturers have mixed their polymeric powder with water and volatile organic liquids so as to produce a flowable liquid Theological product, which can then be poured into aqueous paints, inks and coatings. Environmental concerns, however, have recently made the use of such volatile organic liquids less acceptable, and these liquid thickener compositions have themselves become disfavored. In addition, the use of water and organic liquids, which provide no active rheological properties, increases the weight and bulk of the thickening products which adds substantially to shipping, transportation and packaging costs. Liquid products require expensive non-leakable containers and the added cost of transportation, especially when exporting products, has proved troublesome.
The present invention involves the discovery that a substantially anhydrous mixture or blend of a surfactant, preferrably in powdered form, together with a solid polymeric thickener, defined hereafter, constitutes a product which can easily be dispersed into water-based systems. A wide variety of surfactants have been found effective. The blended inventive product results in a thickening composition with equal or improved viscosity and thickening properties, while avoiding adverse environmental impacts from vapor emissions of volatile organic carriers and the added costs of transportation of now unnecessary liquids.
2. Description of the Prior Art
It has been known in the art for many years to use various materials as Theological additives to, among other rheological properties, modify the viscosity of aqueous systems. Aqueous systems include both water-based and latex emulsion-based paints and coatings, inks, construction materials, cosmetics, and wood stains. Depending on the composition of the aqueous system, the products made with these thickeners can be useful as decorative coatings, paper coatings for magazines, cosmetics and personal care items, adhesives and sealants, numerous types of inks, oil well drilling fluids, construction compounds including joint compounds, and the like.
Natural, modified-natural and synthetic polymers of various types have been utilized as rheological additives. Natural thickeners include mineral clays, casein, and alginates, including vegetable-based gums, such as guar gum. Modified-natural thickeners include modified products using woody plants as their basis, such as cellulosic products, including methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and carboxymethyl cellulose. Such natural and modified-natural products vary widely in their thickening efficiency and their viscosity, flow and leveling modification properties.
Since the 1950s, synthetic polymeric thickeners have assumed increased importance, particularly in the viscosity control of aqueous and latex-emulsion paints and coatings. Synthetic polymeric thickeners are defined to include modified-natural products. Synthetic polymeric thickeners serve several roles in these aqueous systems. When the term, "aqueous systems," is used in this application, it is intended to include latex paints and coatings, as well as latex inks.
In latex and other aqueous paints and coatings, the rheological additive provides improved stability and pigment suspension, and improved use and application properties. In cosmetics and personal care items, such additives improve body, feel, smoothness and silkiness, making the product more aesthetically pleasing and acceptable to sophisticated purchasers. In petroleum drilling fluids, the use of a synthetic thickener improves the suspension of the well-bore cuttings, allows shut-down without fallout of such cuttings, and thereby extends the useful life of the drill head and bit.
Synthetic polymeric Theological thickeners include various acrylic polymers and maleic anhydride copolymers. Two patents issued to Rheox Inc., the assignee of the instant invention, describe a family of solid polyurethane thickening compositions which are today sold under the commercial designation or tradename RHEOLATE. These patents, U.S. Pat. Nos. 4,499,233 and 5,023,309, further describe these synthetic polymeric thickeners as the reaction product of polyisocyanates, polyether polyols and modifying agents (which contain pendant internal hydrophobic groups and at least two active hydrogen moieties) terminated by a chemical capping agent.
Another synthetic polymeric thickener used extensively in commercial paint and coating applications is a low molecular weight polyurethane characterized by hydrophobic groups interconnected by hydrophilic polyether groups, and is disclosed in U.S. Pat. Nos. 4,079,028 and 4,155,892. One similar type of synthetic polymeric thickener is a water-soluble, thermo-plastic organic polymer having a number of monovalent hydrophobic groups incorporated in the internal portion of the polymer molecule. U.S. Pat. Nos. 4,496,708 and 4,426,485, issued to Union Carbide Corporation, describe such thickeners as water-soluble polymers containing pendant internal hydrophobic groups separated from the uncapped end of the polymer by polyether units.
BASF Corporation has been granted a number of patents which disclose polyether Theological additives for aqueous systems obtained by capping a straight-chain polyoxyalkylene compound derived from ethylene oxide and at least one other lower alkylene oxide (a polyoxyalkylene compound initiated with an active hydrogen-containing initiator is disclosed as an example) with an alpha-olefin oxide group. See, for example, U.S. Pat. Nos. 4,288,639, 4,354,956 and 4,904,466. U.S. Pat. Nos. 4,310,436 and 4,709,099 issued to BASF Corporation disclose cross-linked polyesters, polyester-urethanes and polyether-alpha olefins as well as a variety of other types of polymers used as synthetic polymeric thickeners.
Many of the above-described synthetic polymeric rheological additives have often been referred to by the term "associative" rheological additives or thickeners. The mechanism by which such associative thickeners function likely involves "associations" (broadly meaning interactions of some physical or chemical type) between the hydrophobic species in the thickener polymer molecules and other hydrophobic surfaces, either on other thickener polymer molecules, or on molecules in components contained in the system to be thickened. Different commercial types of associative thickeners include polyurethanes, hydrophobically-modified alkali-soluble emulsions, hydrophobically-modified cellulosics, and hydrophobically-modified polyacrylamides.
The molecular weight of these associative thickeners, which also can be referred to as water-soluble synthetic polymers, is sufficiently high enough to impart desired rheological properties to the aqueous composition containing the thickener. Advantageously, the water-soluble polymer operates so that a solution containing up to 2-3 weight percent of the polymer will exhibit a viscosity of at least around 5,000, preferably at least 15,000, and most preferably at least 20,000 centipoises (as measured on a Brookfield viscometer with a number 3 spindle at 10 rpm at 25.degree. C.).
As explained above, a variety of polymeric materials may be used as water-soluble rheological polymers, including cellulose ethers, polyacrylamides, sulfonated polystyrenes, copolymers of acrylic acid, hydroxypropylated guar gum, and the like. The choice of the particular water-soluble polymer depends primarily on the compatibility of such polymer with the other components of the thickening composition that contains the thickener, and the ultimate end use of the aqueous system.
Levels of the rheological thickening polymer additive varying between 0.1% and about 15%, based on the total weight of the system to be thickened, have been found to be useful. As a general class, associative thickeners provide a combination of properties not found in any other single class of known aqueous thickeners. They are generally nonionic, and in many cases are highly efficient viscosity improvers even when they have only middle range molecular weights. They are stable in water, and most are not sensitive to biodegradation.
Associative thickeners can be further defined as molecules containing hydrophobic moieties localized or dispersed through a hydrophilic backbone. The hydrophobic parts cause the association, either with themselves or by association with hydrophobic moieties contained in the various ingredients commonly found in aqueous paints, coatings, and inks. Typical polymer backbones include polyurethanes, polyethers and starch-type molecules. They are versatile in that they both thicken virtually unlimited types of aqueous systems, and also impart many beneficial auxiliary properties. For instance, as additives to textile binder compositions, they may soften rather than harden the fabric. In latex paints they not only thicken but also provide superior flow and leveling properties, and give excellent viscosity control under both low and high shear conditions.
Improvement in flow or leveling as compared to many prior art products, usually accompanied by an increase in viscosity, is observed when the appropriate additive, selected from those described herein, is dispersed in either aqueous or latex systems, or in unpigmented lattices. The beneficial effects are most visible in coating compositions of relatively low pigmentation, or where pigmentation is absent altogether. Such additives often find the widest use in fairly expensive gloss or semi-gloss latex paint compositions.
Water-soluble thickeners for aqueous systems are prepared and have been sold and used in a dry, solid form; a powder form is most common. The dust associated with the incorporation of dry polymers in the industrial environment presents similar conventional material handling problems as are encountered with other types of dry particulates. When added to aqueous systems, moreover, solid powder polymers tend to agglomerate to form clumps. Agglomeration can be reduced in many cases by adding the solid polymer to the aqueous system slowly with agitation. In other cases, the solid thickener is first dispensed in water to form a water/polymer mixture, and then the dispersion is "let down" into the aqueous system to be thickened. Such slow dissolution, however, can affect the efficiency of specific manufacturing operations.
Solid water-soluble polymers have proved particularly difficult to incorporate in industrial paint-making processes, because they often require long periods of time to dissolve. Both in simple aqueous solutions and, more particularly, in formulations comprising other chemicals and ingredients, extended agitation and aging periods are necessary before proper viscosity and dispersion is attained. For the above reasons, manufacturers have searched for a simple, fast and effective way of mixing solid water-soluble polymers into their aqueous systems. Because of this search, many commercial associative thickeners are today sold as pourable liquid "concentrates." Such commercial concentrates, in liquid form for latex paint and other compositions, usually involve taking the solid polymer and preparing a thickening mixture as a blend of water and a water-miscible organic solvent, such as diethylene glycol monobutyl ether (also known as butyl carbitol) or ethylene or propylene glycol. The addition of this organic co-solvent lowers the viscosity of the polymer in water to provide ease in handling, and allows an efficient level of activity in the final product. The water-miscible solvent selected for such commercial compositions has, up to this time, almost exclusively been an organic solvent. For example, commercial polymeric rheological additives for water-borne paint systems, designated Rheox RHEOLATE 255, 278, and 300, are currently sold as liquids in a thickener/butyl carbitol/water mixture.
Although "liquid" rheological additives are used at relatively low levels, as noted above, the use of a volatile organic co-solvent in such rheological additives contributes to the Volatile Organic Content, designated VOC, of the aqueous system to be thickened. The reduction of the VOC of water-borne systems, including important paint and construction systems, without adversely affecting the other properties of the systems, is currently a very active research topic throughout the world, particularly in the United States.
The aforementioned Union Carbide patent, U.S. Pat. No. 4,426,485 describes a solvent/water mixture for a polymer thickening composition of appropriate amounts of polymer, the organic solvent diethylene glycol monobutyl ether, and water. This type of organic solvent evaporates after paints or coatings containing it are applied, and enters the atmosphere during the drying of such aqueous compositions. Similar evaporation occurs following the application of inks, joint compounds and sealants, among other thickened substances.
It has become increasingly important in combating atmospheric pollution to remove organic contaminants, including the vapors released in the course of industrial and architectural painting and in the manufacture of painted and coated articles, such as for example, automobiles and furniture. Such organic vapors not only have an offensive odor, but they may also cause damage to vegetation, wildlife and other aspects of the external environment, besides constituting a health hazard. The authorities in many countries, including the United States and Western Europe, have increasingly imposed ever-lower tolerances as to venting such vapors to the atmosphere.
U.S. Pat. No. 5,425,806, recently issued to Rheox Inc., assignee herein, describes a liquid rheological composition containing an associative thickener and water where butyl carbitol has been replaced with a surfactant to form the liquid pourable additive composition. The patent specification of U.S. Pat. No. 5,137,571, recently issued to Rohm & Haas Corp., exhibits some understanding of the environmental problems which liquid associative thickening compositions can cause. However, the patent continues the practice of mixing a solid polymer into a liquid medium to form a liquid pourable composition. The patent describes the use of starch derivatives of cyclodextrin to suppress the viscosity of an associative thickener in water and create a liquid composition product The cyclodextrin is absorbed onto the hydrophobic moieties of associative thickeners.
U.S. Pat. No. 4,722,962, issued to DeSoto Inc., refers to nonionic surfactants in relation to associative thickeners, but teaches reacting such surfactants into the thickener polymer to form a portion of the final polyurethane. U.S. Pat. No. 5,109,061 describes a surfactant composition comprising a water-soluble polyurethane or polyurea dispersed in a surfactant The composition is prepared by reacting an aromatic or aliphatic polyisocyanate with a polyol or polyamine in the presence of a surfactant, broadly defined as anionic, nonionic, ampholytic, zwitterionic and cationic surfactants. The composition products are disclosed as suitable for preparing molded or shaped articles such as bathtub toys. U.S. Pat. No. 5,270,378 shows acrylic surfactants and compositions (such as graphics systems, including printing plates and imageable polymerizable systems) containing these surfactants. The compositions, in addition, comprise organic solvents.
U.S. Pat. No. 4,432,881 describes thickening agents useful for drilling mud formulations and fracturing fluids containing water-dispersible polyacrylamide and other polymers having pendant hydrophobic groups, as well as cationic and nonionic surfactants, dispersed in an aqueous medium or solution. The hydrophobic groups of the polymer are described as solvated by the surfactant such that the thickening agent forms a solution in water when the two components are agitated with deionized water over a period of time.
Australian Patent No. 515,783, which has no United States counterpart, discloses a thickener concentrate composition which is a mixture of two thickeners, one a nonionic polyurethane and the other a surfactant co-thickener, a non-aqueous inert organic diluent, and from 0 to 65% by weight of water. The surfactant co-thickener is used to increase the viscosity of the print paste into which the concentrate is dispersed. The thickener composition is described as a clear concentrate, and is contrasted with powder thickeners which require subsequent further dilution. The clear concentrates are disclosed as having low manageable viscosity.